As mediums for recording information such as music information, magneto-optical discs are widely used in the form of MDs, for example and are becoming very popular in Japan and foreign countries. Hence, the amount of information recorded on these magneto-optical discs is becoming very large. From a technological standpoint, on the background of the times in which the magneto-optical discs such as the MD have been put on the market at the beginning, an optical system for use with an apparatus for recording and reproducing a magneto-optical disc uses an LD (Laser Diode) having a wavelength of 780 nm as a light source and also uses an objective lens having a numerical aperture NA of 0.45.
Also, in the magneto-optical recording format such as the ISO, there have been proposed a method in which an information recording mark is recorded on a guide groove formed on the substrate of a magneto-optical recording medium, i.e. so-called groove and a method in which an information recording mark is recorded on a land formed between the grooves. In addition, Official Gazette of Japanese laid-open patent application No. 10-320780, etc. have proposed a method in which an information recording mark is recorded on both of a land and a groove. In the MD, information is recorded on the groove, and a distance between the grooves, that is, a track pitch Tp is selected to be approximately 1.6 μm.
It has been requested that such MD should be modified into the system capable of recording a moving picture and the like from an accessibility standpoint. The most important point of the points at which the system should be modified is to increase a recording density of a magneto-optical disc. While music information needs a recording density of approximately 100 MB, a moving picture requires a recording density of at least about 10 times as high as the above-mentioned recording density depending upon image quality. Although various methods have been so far proposed in order to achieve these requirements, one of such previously-proposed methods is a magnetically induced super resolution reproducing system, that is, a so-called MSR (Magnetically induced Super Resolution) system that is proposed in Japanese Patent No. 2805746, for example.
The MSR system will be described below in brief. According to this technology, a recording and reproducing film of a magneto-optical disk is comprised of a plurality of magnetic layers having proper coercive force, proper exchange-coupling force, a Curie temperature and the like, for example, a recording layer and a reproducing layer or an intermediate layer interposed between the recording layer and the reproducing layer. This technology uses the fact that a temperature produced on the recording and reproducing film on the magneto-optical disc with irradiation of reproducing laser light changes depending on the positions within the irradiated spot. Magnetization of the recording layer is transferred to the reproducing layer only in a certain limited temperature region. In the temperature region outside this temperature region, regardless of magnetizations of the recording layer, the magnetizations of the reproducing layer are arrayed in one direction, for example, to produce a magnetic mask on a part of the inside of a so-called irradiated spot. Thus, even when a plurality of marks is formed within the spot, it becomes possible to reproduce a part of the recording marks, thereby improving a resolution.
With respect to the above-mentioned MSR system, various systems have been proposed, which will be described below.
Official Gazette of Japanese laid-open patent application No. 1-143042 has proposed a so-called FAD (Front Aperture Detection) system which detects recording marks located ahead of the direction in which the irradiated spot is moved. According to the FAD system, a magneto-optical recording layer is composed of magnetic layers of a tri-layer structure comprising a reproducing layer made of GdFeCo or the like, an intermediate switching layer made of TeFeCoAl or the like and a recording layer made of TeFeCo or the like. When a laser spot is irradiated on a rotating disc-like medium, a high temperature region is slightly displaced rearwards from the center of the spot due to heat conductivity. In the high temperature region within this spot, since the temperature of the intermediate switching layer rises in excess of the Curie temperature, the exchange-coupling force between the reproducing layer and the recording layer decreases so that magnetizations of the reproducing layer selected to be the material with small coercive force are arrayed by reproduced magnetic fields and thereby information is erased, that is, masked. As a result, only the magnetizations of the recording marks of the front portion which is the low temperature region can be detected in the state in which they were transferred to the reproducing layer, and hence super resolution becomes possible.
Also, Official Gazette of Japanese laid-open patent application No. 5-81717, Official Gazette of Japanese laid-open patent application No. 5-12731 and the like, for example, have proposed a so-called CAD (Center Aperture Detection) system in which magnetization of a reproducing layer changes from surface magnetization to perpendicular magnetization only in the high temperature region at the central portion of the irradiation spot to read only recording marks from this portion.
Further, Official Gazette of Japanese laid-open patent application No. 3-90358, Official Gazette of Japanese laid-open patent application No. 4-271039 and the like, for example, have proposed a so-called RAD (Rear Aperture Detection) system for detecting rear recording marks of the spot.
On the other hand, Official Gazette of Japanese laid-open patent application No. 4-255946 and Official Gazette of Japanese laid-open patent application No. 4-271039 and the like, for example, have proposed a so-called D-RAD (Double mask Rear Aperture Detection) system in which a magneto-optical recording layer is composed of a recording layer, an intermediate layer and a reproducing layer. Upon reproduction, in the state in which the reproducing layer is magnetized in one direction, a reproducing magnetic field is applied to the reproducing layer along the magnetization direction to produce a low temperature region, a reproducible region and a high temperature region in the region irradiated with the illumination spot, a sum of the reproducing magnetic field and coercive force of the reproducing layer becomes small only in the reproducible region as compared with a magnetic field produced by a magnetic wall between the reproducing layer and the intermediate layer formed just under the reproducing layer, whereby the magnetization of the recording layer in this reproducible region is transferred to the reproducing layer to thereby reproduce information by a magnetically induced super resolution system.
An optical recording medium using this D-RAD system has already been commercially available on the market as an optical recording medium called “GIGAMO” (trade name and produced by Sony Corporation).
Further, Official Gazette of Japanese laid-open patent application NO. 6-290469 and the like, for example, have proposed a so-called DWDD (Domain Wall Displacement Detection) system in which a reproducing layer is made of a material of which domain wall coercive force is small and whose domain wall displacement degree is large as compared with those of a recording layer, a Curie temperature of an intermediate layer between the reproducing layer and the recording layer is selected to be small as compared with those of the reproducing layer and the recording layer and the magnetic domain of the reproducing layer is enlarged in the intermediate layer within the irradiated spot at its high temperature region of which temperature rises in excess of the Curie temperature to read out the magnetization of the recording layer.
Furthermore, Official Gazette of Japanese laid-open patent application No. 8-7350 and the like, for example, have proposed a so-called MAMMOS (Magnetic Amplifying MO System) system in which a magnetic domain of a recording layer is transferred to a reproducing layer by effectively utilizing an external magnetic field and in which the magnetic domain transferred to this reproducing layer is enlarged to read out the magnetization of the recording layer.
On the other hand, there are requests of effectively utilizing and keeping property such as information diffused on the whole world by magneto-optical discs such as the conventional MD, and the appearance of an apparatus or a method or a medium using this apparatus and method that can use moving pictures while the property of the above information can be used has been desired. That is, while keeping compatibility with the existing MD, the above medium is requested to increase only a recording density approximately 10 times.
It has been customary for the MD recording and reproducing apparatus to use the optical system using a light source with a wavelength selected to be 780 nm and an objective lens with a numerical aperture NA selected to be 0.45 as described above. Similarly to other optical discs, a shape of a focused beam spot is substantially determined by a wavelength of a light source and a numerical aperture of an objective lens.
After the shape of the beam spot is determined, the track pitch that can be used in that spot and the shortest pit length corresponding to a linear density are determined. In the case of the MD, the track pitch is substantially 1.6 μm, and the shortest bit length is 0.59 μm. The shortest bit length is determined by an MTF (Modulation Transfer Function: modulation transfer function) at the determined spot and the modulation-and-demodulation. With this bit length, when this optical system is used, it is possible to obtain a sufficient reproduced signal.
Also, a track pitch is selected in a range in which tracking can be achieved by the determined spot. Tracking uses a difference between reflectivities between the land and the groove, and not only the track pitch conditions but also the groove depth condition are added. A report entitled “The Main Point of Setting of Optical Pickup System” (compiled under the supervision of Mr. Noda, Electronics Essential Series No. 6, Japan Industry Engineering Center, 1984) has described the study that when a wavelength of light is λ and a refractive index of a substrate is n, then a signal is lost if the groove depth is set to be λ/4 n.
Although the track pitch Tp and the shortest bit length are determined as described above, both of them should be decreased because of the demand of increasing a recording density. It is possible to decrease the track pitch to about 1.2 μm in a range in which a reproducing signal used to apply tracking can be held stably.
However, if the track pitch is reduced to 1.2 μm, then phenomenon in which upon reproduction, the adjacent track is affected by heat and information is recorded on the adjacent track, that is, so-called cross-write will occur unavoidably.
Although the cross-write is not a serious problem when a recording mark is not formed on the adjacent track, if information is recorded on the adjacent track, then such recorded information is broken, and hence the cross-write is a problem that should be prevented from a reliability standpoint.
It is an object of the present invention to provide an optical recording and reproducing method and an optical recording medium for use with the above optical recording and reproducing method in which the above-mentioned problems can be solved, which have compatibility with existing and most widely-used magneto-optical disc such as an MD, in which high density recording is possible, that is, a recording capacity is increased to make recording and reproduction of a moving picture become possible. Further, it is another object of the present invention to provide an optical recording and reproducing method and an optical recording medium for use with the above optical recording and reproducing method in which the occurrence of cross-write can be avoided and which is excellent in recording and reproducing characteristic.